DESCRIPTION

These eleven functions were defined in C99, and describe the handling
of floating point rounding and exceptions (overflow, zero-divide etc.).

Exceptions

The DivideByZero exception occurs when an operation on finite numbers
produces infinity as exact answer.

The Overflow exception occurs when a result has to be represented as a
floating point number, but has (much) larger absolute value than the
largest (finite) floating point number that is representable.

The Underflow exception occurs when a result has to be represented as a
floating point number, but has smaller absolute value than the smallest
positive normalized floating point number (and would lose much accuracy
when represented as a denormalized number).

The Inexact exception occurs when the rounded result of an operation
is not equal to the infinite precision result.
It may occur whenever Overflow or Underflow occurs.

The Invalid exception occurs when there is no well-defined result
for an operation, as for 0/0 or infinity - infinity or sqrt(-1).

Exception handling

Exceptions are represented in two ways: as a single bit
(exception present/absent), and these bits correspond in some
implementation-defined way with bit positions in an integer,
and also as an opaque structure that may contain more information
about the exception (perhaps the code address where it occurred).

Each of the macros
FE_DIVBYZERO,
FE_INEXACT,
FE_INVALID,
FE_OVERFLOW,
FE_UNDERFLOW
is defined when the implementation supports handling
of the corresponding exception, and if so then
defines the corresponding bit(s), so that one can call
exception handling functions e.g. using the integer argument
FE_OVERFLOW|FE_UNDERFLOW.
Other exceptions may be supported. The macro
FE_ALL_EXCEPT
is the bitwise OR of all bits corresponding to supported exceptions.

The
feclearexcept
function clears the supported exceptions represented by the bits
in its argument.

The
fegetexceptflag
function stores a representation of the state of the exception flags
represented by the argument
excepts
in the opaque object
*flagp.

The
feraiseexcept
function raises the supported exceptions represented by the bits in
excepts.

The
fesetexceptflag
function sets the complete status for the exceptions represented by
excepts
to the value
*flagp.
This value must have been obtained by an earlier call of
fegetexceptflag
with a last argument that contained all bits in
excepts.

The
fetestexcept
function returns a word in which the bits are set that were
set in the argument
excepts
and for which the corresponding exception is currently set.

Rounding

Each of the macros
FE_DOWNWARD,
FE_TONEAREST,
FE_TOWARDZERO,
FE_UPWARD
is defined when the implementation supports getting and setting
the corresponding rounding direction.

The
fegetround
function returns the macro corresponding to the current
rounding mode.

The
fesetround
function sets the rounding mode as specified by its argument
and returns zero when it was successful.

Floating point environment

The entire floating point environment, including
control modes and status flags, can be handled
as one opaque object, of type
fenv_t.
The default environment is denoted by
FE_DFL_ENV
(of type
const fenv_t *).
This is the environment setup at program start and it is defined by
ISO C to have round to nearest, all exceptions cleared and a non-stop
(continue on exceptions) mode.

The
fegetenv
function saves the current floating point environment in the object
*envp.

The
feholdexcept
function does the same, then clears all exception flags,
and sets a non-stop (continue on exceptions) mode,
if available. It returns zero when successful.

The
fesetenv
function restores the floating point environment from
the object
*envp.
This object must be known to be valid, e.g., the result of a call to
fegetenv
or
feholdexcept
or equal to
FE_DFL_ENV.
This call does not raise exceptions.

The
feupdateenv
function installs the floating-point environment represented by
the object
*envp,
except that currently raised exceptions are not cleared.
After calling this function, the raised exceptions will be a bitwise OR
of those previously set with those in
*envp.
As before, the object
*envp
must be known to be valid.

GNU DETAILS

If possible, the GNU C Library defines a macro
FE_NOMASK_ENV
which represents an environment where every exception raised causes a
trap to occur. You can test for this macro using
#ifdef.
It is only defined if
_GNU_SOURCE
is defined.
The C99 standard does not define a way to set individual bits in the
floating point mask, e.g. to trap on specific flags.
glibc 2.2 supports the functions
feenableexcept
and
fedisableexcept
to set individual floating point traps, and
fegetexcept
to query the state.

The
feenableexcept
and
fedisableexcept
functions enable (disable) traps for each of the exceptions represented by
excepts
and return the previous set of enabled exceptions when successful,
and -1 otherwise.
The
fegetexcept
function returns the set of all currently enabled exceptions.